Drilling (Boring) a hole in concrete, stone, etc, is done by attaching a dedicated drill bit to a rotating hammer drill and by applying both an axial vibrational striking force and a rotational torque to the drill bit. This type of drill bit is constructed such that a cutting blade body made of wear-resistant cemented carbide is fixedly attached to a tip end of a steel bit body by brazing, welding or the like. In addition, chip discharge grooves are provided on an outer periphery of the bit body so as to correspond to concave portions formed on cutting blades of the cutting blade body. Further, a standardized shank portion is attached to a base end portion of the bit body and configured to be coupled to a drive shaft of the hammer drill.
The applicant disclosed a novel drill bit of this type (Japanese Patent Application No. 2000-377082). This drill bit is constructed such that a plurality of cutting blades extend substantially radially outward from the center of a block-like cutting blade body fixedly attached to a tip end of a bit body. Each cutting blade has a face and a flank. An edge formed at a boundary between the face and the flank of the cutting blade to protrude toward a tip end of the cutting blade body to form a cutting edge. And, inner ends (ends on the center side) of the cutting blades are coupled to form the tip end pointed without a chisel edge. Between adjacent cutting blades, a chip discharge groove is formed by a concave portion recessed backward. The chip discharge groove is continuous with a wide bit chip discharge groove provided on an outer periphery of the bit body.
In accordance with the drill bit constructed as described above, during drilling the hole in the concrete, the stone, etc, cutting (striking and cutting) is performed in such a manner that the cutting blade impactly strikes a cutting face of a drill hole, thereby generating chips. These chips move up from the cutting face of the drill hole according to an axial movement of the drill bit, and moves through each chip discharge groove. Then, the chips are discharged into the wide bit chip discharge groove. As should be appreciated, the chips are smoothly discharged outside the drill hole. Thus, regardless of a depth of the drill hole, the chips are efficiently discharged. Further, since the tip end of the cutting blades is pointed without the chisel edge, the center of rotation is uniquely determined in drilling, thereby allowing the cutting blades to stably rotate. As a consequence, it is possible to easily drill a hole having a desired diameter and a desired shape. In addition, highly durable drill bit is achieved.
However, in the above-constructed drill bit, the cutting blades of the cutting blade body are configured (structured) in the same manner. Because of this, when the cutting blade strikes hard object such as reinforcing steel within concrete or stone during drilling the hole in the concrete or the like, an impact force concentratively acts on the cutting blade that has struck the object or its adjacent cutting blade, thereby causing a great change in a drilling condition.
During drilling, various sizes of large and small chips, including large chip lumps, are generated on the cutting face of the drill hole. Since the large chip lumps are crushed into small chips during repeated striking and cutting, drilling is not impeded by the large chip lumps. However, while the large chip lumps are being struck and crushed by the cutting edge of the drill bit, the cutting edge does not perform a cutting action for the cutting face of the drill hole, that is, the cutting action is interrupted. This reduces drilling efficiency.